The present invention is directed to a method and an arrangement for measuring a magnetic field.
Optical measuring arrangements and measuring methods for measuring a magnetic field, using the magneto-optic Faraday effect, are known. The Faraday effect is the term used to describe the phenomenon in which a plane of polarization of linearly polarized light is rotated as a function of a magnetic field. The angle of rotation is proportional to the path integral over the magnetic field along the path traced by the light, with the "Verdet constant" as a constant of proportionality. The Verdet constant is generally dependent on several factors, including the material, the temperature, and the wavelength. To measure the magnetic field, a Faraday sensor device made of an optically transparent material such as, for example, glass, is arranged in the magnetic field. The magnetic field effects a rotation of the plane of polarization of linearly polarized light transmitted through the Faraday sensor device. The angle of rotation can be evaluated using a measuring signal.
Such magneto-optic measuring methods and measuring arrangements find application, as is known, in the measurement of electric currents. The Faraday sensor device is arranged for this purpose in the vicinity of a current conductor and detects the magnetic field produced by a current in the current conductor. In general, the Faraday sensor device surrounds the current conductor, so that the measuring light orbits around the current conductor in a closed path. The magnitude of the angle of rotation in this case is, to a good approximation, directly proportional to the amplitude of the current to be measured. The Faraday sensor device can be designed as a solid glass ring around the current conductor, or, alternatively, the sensor device can surround the current conductor in the form of a measurement winding made of a light-guiding fiber (fiber coil) having at least one turn.
Advantages of these magneto-optic measuring arrangements and measuring methods over conventional inductive current converters are the potential isolation and the insensitivity with respect to electromagnetic disturbances. However, magneto-optic current converters are vulnerable to mechanical vibrations, which can affect the sensor device and the optical feed lines and consequently lead to intensity changes that render the measurement as inaccurate. The sensor device is also vulnerable to temperature changes.
In order to reduce the influence of vibrations on the measurement, previously proposed systems have transmitted two oppositely directed light signals, that is to say, light signals propagating in opposite directions, through a Faraday sensor device. This measure is based on the idea that the linear birefringence experienced by the two light signals on their common light path as a result of vibrations can be differentiated as a reciprocal effect from the nonreciprocal Faraday effect by means of suitable signal processing.
In one of these previously proposed systems, two linearly polarized light signals, running in opposite directions, are transmitted through an optical fiber coil as a Faraday sensor device which surrounds a current conductor. Provided as optical fiber for the fiber coil is a mechanically twisted fiber or a fiber twisted during the drawing process and having a high linear birefringence (spun HiBi fiber). Apart from the Faraday effect, the optical fiber also exhibits a circular birefringence which is high in comparison with the Faraday effect. After passing through the sensor device, each of the two light signals is decomposed by a polarizing beam splitter into two components which are polarized transversely to each other. Using the total of four light components, a measuring signal is derived by a signal processing means for an electric current in the current conductor. The signal essentially corresponds to the quotient of the Faraday measuring angle and the circular birefringence of the fiber and is therefore independent of a linear birefringence in the optical fiber. Although the measuring signal thus obtained is largely free of temperature-induced linear birefringence in the sensor device, the measuring signal is still temperature-dependent because of the temperature dependence of the circular birefringence of the fiber. In this system, the two light signals, running in opposite directions, pass through only the Faraday sensor device on a common light path and, upon emerging from the Faraday sensor device, are again separated from each other by optical couplers. This system is described in PCT publication WO 92/13280.
Other previously proposed systems use two light signals that pass, in mutually opposite directions of circulation, through an optical series circuit consisting of a first optical fiber, a first polarizer, a Faraday sensor device, a second polarizer, and a second optical fiber. Both light signals, after passing through the optical series circuit, are converted in each case by corresponding photoelectric transducers into one electric intensity signal.
In U.S. Pat. No. 4,916,387, a solid glass ring which surrounds the current conductor is provided as a Faraday sensor device. The axes of polarization of the two polarizers are rotated at an angle of 45.degree. relative to each other. To compensate for undesired intensity changes in the optical feed fibers, it is assumed that the undesired intensity changes (noise) and the intensity changes caused by the Faraday effect are cumulatively superimposed with different signs in the two electric intensity signals, and can therefore be separated from each other.
In a previously proposed system discussed in the Journal of Lightwave Technology, Vol 12. No. 10, October 1994, pages 1882 to 1890, a fiber coil made of a single-mode fiber with a low birefringence is provided as the Faraday sensor device. The axes of polarization of the two polarizers mutually form a polarizer angle that is different from 0.degree., which is preferably 45.degree.. Light from a single light source is split into the two light signals, and these light signals are each coupled via an optical coupler into the associated optical fiber. From the two electric intensity signals which correspond to the light intensities of the associated light signals after passing through the series circuit, a measuring signal is derived which corresponds to the quotient of the difference and the sum of the two intensity signals. Hence, the attenuation factors of the two optical fibers can essentially be compensated. Nevertheless, the light intensities of the two light signals must be adjusted to be exactly equal when coupling into the series circuit.
In another previously proposed system, which is discussed in "Transmission loss compensation for Faraday effect fiber optic sensors", Conference Eurosensors VIII, Toulouse, Sep. 25-28, 1994, an optical series circuit, which consists of multimode fibers as optical fibers, polarizers and Faraday sensor device is connected between two infrared light-emitting diodes. The two light-emitting diodes are operated alternately as light source and as photodetector. Thus, only one of the two oppositely directed light signals is ever passing through the series circuit at one point of time. Therefore, the changeover clock frequency is selected to be as high as possible.